Binding isotherm, cationic surfactant

An electrode selectively responsive to an ionic ligand in equilibrium with ligands bound on a macromolecule can be utilized to construct binding isotherms.21 23,35 Dodecylpyridinium halide was chosen in this work as an amphiphilic ligand because an excellent electrode sensitive to it was found. This amphiphile is regarded as a model compound of many classes of biomedical molecules since it combines hydrophobic, aromatic, and polar (cationic) groups. The limited number of papers published on DNA-surfactant interaction,36 further prompted this work. 

The fact that there are more investigations of binding isotherms of cationic surfactants by anionic polymers than vice-versa undoubtedly reflects the fact (71) that it has been found easier to home-make electrodes for these surfactants than for anionic surfactants. (See, however, Chapter 10 and Section 11.A.2 above.) A typical electrode, employing a cell with aPVC membrane impregnated with a 1 1 complex of DTAB and SDS, showed Nernstian response to DTAB over three decades of concentration (18,19). 

If a cationic surfactant is progressively added to a DNA solution, one will observe that initially there is no significant interaction. However, at a quite well-defined concentration the surfactant activity, equaling the concentration of free surfactant, becomes rather independent of the surfactant concentration, signifying an association of surfactant to DNA [8-10]. The binding isotherm that is obtained is illustrated in Figure 10.2. 

Interestingly, the cac is much lower when DNA is denaturated, and in the single-stranded conformation, than for the double-helix DNA, as illustrated by binding isotherms (curve 1 vs. curve 2 in Figure 10.2) [10] and in a recent study that deduced the cac from conductivity data [14]. This is a simple example of a stronger DNA-cationic surfactant interaction for ss- than for ds-DNA. 

The effect of simple salt on the surfactant binding confirms the electrostatic nature of the surfactant ion-polyion interactions (step 1, Scheme 1). Any increase in the ionic strength of solution shifts the onset of binding toward higher free surfactant concentrations (compare isotherms in water, 0.01 M, and 0.1 M NaCl, Figure 6) and decreases the amount of bound surfactant. These observations can be related to the screening influence of the simple salt, which acts to diminish the electrostatic interactions between surfactant cations and polyanions. This is also well documented in the literature for a variety of polyelectrolyte-surfactant pairs [26-29],... 

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